export interface Sum<A, B, C> {
  <D, E, F>(a: A, b: A): void;
}

type MySum = Sum<number, number, number>;
let sum: MySum = <B>(a: number, b: number) => {};
//写在参数前面的是在调用的时候用到了
sum<number, number, number>(1, 2);
